The invention relates to elastomeric compositions suitable for use in the tread portion of a pneumatic tire, to a process for the preparation of said elastomeric compositions, to unvulcanized elastomeric compositions containing said elastomeric compositions and to tires containing the elastomeric compositions in a vulcanized state.
Tires containing elastomeric compositions in a vulcanized state should have a low rolling resistance for low fuel consumption, a high wet grip performance for good running safety of automobiles and a high resistance against abrasion. Two general classes of tread compounds can be distinguished:
(a) those giving high wet grip performance combined with high heat build-up characteristics, relatively high rolling resistance, a low resistance against abrasion and having a glass transition temperature above minus 55.degree. C., and PA0 (b) those giving relatively low wet grip performance combined with low heat build-up characteristics, relatively low rolling resistance, a high resistance against abrasion and having a glass transition temperature below minus 65.degree. C. PA0 (1) in the range of from 10 to 90% by weight of a copolymer A of a mono-aromatic vinyl compound and a conjugated diene and having a content of mono-aromatic vinyl compound in the range of from 10 to 40% by weight, a vinyl content in the conjugated diene units of at least 50% by weight, calculated on total conjugated diene units, and a glass transition temperature above minus 35.degree. C., and PA0 (2) in the range of from 90 to 10% by weight of a polymer B which is a homopolymer of a conjugated diene or a copolymer of a mono-aromatic vinyl compound and a conjugated diene, said polymer B containing less than 25% by weight of mono-aromatic vinyl compound and having a vinyl content in the conjugated diene units of less than 25% and a glass transition temperature below minus 55.degree. C.,
From the above it follows that it is difficult to combine a low rolling resistance with a high wet grip performance.
British patent specification No. 2,068,979 concerns rubber compositions comprising (A) from 10 to 90% by weight of a random styrene-butadiene copolymer having a bound styrene content of from 3 to 30% by weight and a butadiene portion which has a 1,2 content of from 60 to 95% by weight, said copolymer being obtained by polymerization in the presence of an organo-alkali metal catalyst, and (B) from 90 to 10% by weight of at least one rubber having a glass transition temperature not higher than minus 40.degree. C., said rubber being natural rubber, polyisoprene rubber, styrene-butadiene copolymer rubber or polybutadiene rubber.
"Random" according to British patent specification No. 2,068,979, refers to a styrene-butadiene copolymer, the bound styrene of which contains 10% by weight or less of block styrene. The random styrene-butadiene copolymer (A) can be prepared, for example, by polymerizing styrene and butadiene in a hydrocarbon solvent in the presence of an additive such as an ether or a tertiary amine and using an organolithium compound as initiator. Pneumatic tire treads made from such compositions have improved wet skid and heat build-up properties.
U.S. Pat. No. 4,485,205 concerns elastomeric composions suitable for use in the tread portion of a pneumatic tire. These known compositions comprise (1) a butadiene-styrene copolymer rubber containing 3-30% by weight of bound styrene and having a vinyl content in the conjugated diene units of not less than 60% by weight and a glass transition temperature of -35.degree. C. to 0.degree. C., and (2) a diene rubber having a glass transition temperature of lower than -60.degree. C. The two components are not compatible, i.e. the glass transition temperatures of these rubbers appear separately, i.e. they are not united.
According to Example 1 in said specification the copolymers were prepared by polymerization using n-butyllithium as a catalyst and potassium dodecylbenzenesulphonate and diethylene glycol dimethyl ether as a modifier in a toluene solvent at constant temperature. Pneumatic tire treads made from such compositions have a low rolling resistance, a high wet grip performance and a high resistance against abrasion.
A disadvantage of the tires manufactured from the known elastomeric compositions described hereinbefore is that their winter performance is relatively poor, which follows from their relatively high Shore hardness at a temperature of minus 10.degree. C. The known tires, therefore, have a relatively poor grip on snow and ice.